As a method of extracting alumina from an alumina-containing ore, the Bayer process is generally adopted. In the Bayer process, generally as the alumina-containing ore, bauxite, which is so called because of the geographical name of the origin thereof, after wet-grinding, is mixed with an alkaline solution to obtain slurry; the slurry is treated at high temperature so as to extract an alumina component that is contained in bauxite; after that, bauxite residue (extraction residue: usually called as “red mud” because of its red color provided from an iron component and humin contained therein) that is an undissolved component is subjected to solid-liquid separation. Aluminum hydroxide is precipitated out of an alumina-extracted liquid (aluminate liquor) from which red mud has been removed. Then, the aluminum hydroxide is calcinated so as to obtain alumina.
Separation of the residue (red mud) that is the undissolved component of bauxite is performed by cooling, at around 100° C. under atmospheric pressure, the extracted slurry that has been heat-treated at high temperature. As a separation apparatus, usually a thickener equipped with a mud collector is adopted. The solid-liquid separation requires long time because the particle size of the residue is extremely small as 10 μm or less. Therefore, in order to promote settling of the bauxite residue, conventionally a water-soluble polymer flocculant is used. In Patent Document 1, as the polymer flocculant, sodium acrylate, a copolymer of sodium acrylate and amide acrylate, or the like is used. In this document, in order to improve settling rate of the residue and to enhance the level of concentration to solid of the lower liquid of the thickener and the clarification of the upper liquid, a flocculant including starch and slaked lime is also used. Patent Document 1 further describes that alumina-containing ores produced from Australia and Jamaica are used, wherein the polymer flocculant used in the initial process of an alumina recovery circuit is different from the polymer flocculant used in the following process.
However, these polymer flocculants are not always effective for the separation of extraction residues obtained from all of the alumina-containing ores. For example, it is known that these are effective for promoting settling of the red mud obtained from bauxite produced from GOVE ore in Australia, but these are not effective for the red mud obtained from bauxite produced in Southeast Asia, particularly in Indonesia. This is because, while bauxite residue contains mainly such components as iron, silica, aluminum, titanium, and sodium, these components have different crystal morphologies depending on the kinds of bauxite, whereby settling rate, level of concentration to solid, clarification, and others are affected. Examples of compositions that exhibit an adequate settling rate include hematite (Fe2O3), anatase (TiO2), and boehmite (γ-AlOOH). On the other hand, examples of compositions that exhibit poor settling rate include goethite (FeO(OH)), sodalite (Na4ClSi3Al3O12), rutile (TiO2), and gibbsite (Al(OH)3) (Non-Patent Document 1).
Alumina-containing ores represented by BINTAN ore and TAYAN ore of Indonesia are imported in a large amount to Japan. These ores contain a trihydrate (gibbsite) as an alumina component and FeO(OH) (goethite) or (Fe,Al)O(OH) (goethite aluminum) as an iron component. When such ores are treated by conventional Bayer process, because goethite that has a slow settling rate is contained in the extraction residue (red mud), settling separation with a conventional flocculant is difficult. In view of the above circumstances, there have been various kinds of proposals to improve settling property of such bauxite extraction residues that have a high content of goethite.
Patent Document 2 describes a process of converting goethite that is contained in bauxite into magnetite (Fe3O4) by rising alumina extraction temperature. In this process, alumina extraction is performed in the presence of a reducing organic substance and at an extraction temperature of 270° C. or higher so as to convert goethite into magnetite that has an adequate settling property, whereby settling is improved. This process has a problem of being not economically advantageous in view of energy consumption or requiring expensive materials for apparatuses.
Patent Document 3 and Patent Document 4 describe a process of combining substances that have an adequate settling property. In the process, a substance that contains a composition having an adequate settling property is added and mixed, so that the composition ratio in the residue is changed and that substances having poor settling property are separated adequately. However, in the process, the composition having an adequate settling property is required to be ready for use at any time, whereby the process becomes complicated and is not economically advantageous.
Furthermore, Patent Document 5 describes a proposal of using starch that is cationized into a quaternary ammonium type as a flocculant. Settling property is somewhat improved by using the flocculant for the separation of red mud obtained from an alumina-containing ore that involves goethite therein, however, it is considered not to be sufficient for an industrial treatment.
As described above, in the red mud separation treatment for alumina-containing ores that are imported into Japan in a large amount, particularly the ores that are produced in Indonesia and involve goethite therein, a flocculant that has both adequate settling separation performance and economical advantages and exhibits sufficient practical performance by itself has not been known so far.
In view of the above circumstances, Patent Document 6 describes a proposal of using a water-soluble polymer flocculant that contains, as an effective ingredient, a copolymer having a repeating unit derived from a vinylhydroxamic acid compound in the separation treatment of red mud obtained from alumina-containing ores that involve goethite therein.